The measurement of sulfur in the parts per billion (ppb) levels is particularly relevant for liquefied petroleum gas, refinery gas or natural gas (collectively industrial hydrocarbon products). Industrial hydrocarbon products contaminated with even ppb levels of Sulfur can poison catalysts used in industrial hydrocarbon product processing and various fuel cell technologies are sensitive to similar low Sulfur levels. The problem to be solved is the provision of a stable reference gas for use as a calibration and analysis standard in the ppb range.
The state of the art includes some solutions to the problem of ppb Sulfur standard stability. One approach is to perform a sophisticated passivation process which significantly limits the loss of Sulfur to adsorption by container walls. These technologies are provided for many reactive gases under the brand ALPHATECH™. ALPHATECH™ is described in various U.S. patents including U.S. Pat. No. 7,837,806. ALPHATECH™ requires a sophisticated and specialized manufacturing base and this reality translates into a relatively high priced calibration gas product.
A second alternative approach is to avoid the problem by using higher concentration and more stable Sulfur standards to derive a diluted ppb standard on demand.
One example of an on-site dilution approach is the use of permeation tubes to generate low concentration reference gas. U.S. Pat. No. 6,722,182 describes an example of this approach. Permeation tubes generally require precise carrier gas flow control and temperature control. The provision, maintenance and operation of permeation tubes require special software and process integration into an industrial facility that have made use of this technology difficult. {Not adopted in hydrocarbon industrial analysis because of difficulty in using.}
In response to the need for a better solution to the problem, AGILENT™ developed an on site, high precision, blending system. This system is designed to dilute a 5 ppm Sulfur standard down to as low as 20 ppb. R. L. Firor and B. D. Quimby, Automated Dynamic Blending System for the Agilent 6890 Gas Chromatograph: Low Level Sulfur Detection, Publication Number 5988-2465, April 2001. The blending approach has the advantage of using cylinder gas as the source for a diluted ppb Sulfur standard. As with permeation tubes, blending systems still require integration of new equipment into existing industrial settings and processes. The high precision blender technology is also still relatively expensive.
While the art has provided a number of solutions to the problem of providing a reliable ppb level Sulfur gas standard, these solutions all require sophisticated technology that results in substantial cost. Thus, in many circumstances, the only practical option is to prepare fresh standard gases and use these sulfur standards immediately, particularly for ppb level measurements. See, e.g., Sudhir Kumar Pandey, Ki-Hyun Kim, Comparison of different calibration approaches in the application of thermal desorption technique: A test on gaseous reduced sulfur compounds, Microchemical Journal, Volume 91, Issue 1, January 2009, Pages 40-46, ISSN 0026-265X, 10.1016/j.microc.2008.07.004. There is therefore a need in the art for a solution that is both less expensive and less complex in nature. Ideally, the solution would be cylinder gas based such that the gas standard may be pre-certified by the calibration gas vendor. The present invention provides such a solution.